Generally known sensors for measuring gases and vapors are optical filters containing a sensor material which reversibly changes color in the presence of a gas or vapor. This color change affects the transmittancy of the filter under the influence of the gases or vapors. These filters contain a mixture of an alkaline, or acid, color former, also known as a colorant, and a complementary compound. Triphenylmethane compositions, preferably crystal violet lactone, for example, can be utilized as color formers ("colorants"). These filters may also consist of colorants of the triphenylmethane system, preferably phthalein or sulphophthalein, which can be embedded in a matrix and provided with a carrier. The change in the transmittancy of the filter, under the effect of the gases or vapors, is converted into an electric signal and processed electronically. A filter such as generally described above is discussed in German Published Patent Application No. 35 06 686.
Metal complexes having ligands with hydrophobing properties are generally known. Examples of these metal complexes include: monodentate ligands, for example dimethyl formamide; bidentate ligands; chelate ligands, for example ethylenediamine and acetylacetone, podandens and macrocylenes such as crown ethers and cryptands.
A change in the electrical properties, for example a change in the dielectric constant or the electrical conductivity of a material can be utilized to measure, or sense, gases or vapors. For example, Sensorik, Springer Publishers, Heidelberg, 1986, pages 166-170 discusses the use of an ionic conductor as a sensor for oxygen or other gases. The ionic conductor separates two gas compartments, with different partial oxygen pressures, from each other. Porous platinum is applied to both sides of the ionic conductor and combines with the gas to be measured to form an electrode. If a difference exists between the gas pressures on the two sides of the ionic conductor, then a potential difference develops between the platinum electrodes which have formed on each side of the ionic conductor. This change in the electrical properties, as a result of the effect of the gas, can be used as a measured quantity to sense measure or sense gas or vapors.
Sensors, utilized to selectively determine components in a liquid or gaseous phase, having an MOS (metal oxide semiconductor) structure, such as a field-effect transistor or an MOS diode, are discussed in German Published Patent Application No. 35 36 348. These sensors are well suited to qualitatively or quantitatively determine analytes, for example molecules or ions, in liquid or gases. In these sensors a semi-conductive duct develops between a drain and a source electrode which are formed by the doped surface areas of the semi-conductor substrate. In the area of this duct, the substrate is covered by an insulating layer of silicon dioxide SiO.sub.2. The silicon dioxide layer may additionally be covered by a thin protective layer of silicon nitride Si.sub.3 N.sub.4. An additional gate insulating layer serves as a sensor layer for the gases to be measured and is arranged under a gate electrode, which is permeable to gas. This sensor layer with a thickness of approximately 0.01 to 1 .mu.m consists of heteropolysiloxane. Heteropolysiloxanes are silicates which are organically modified by incorporating functional groups and, through reactions with other equally hydrolyzable and condensable metal compounds. By incorporating functional groups, for example primary amino groups, absorption centers develop which enable the desired selective interaction with analytes. As a result of the change in the electrical properties of the sensor layer and, possibly, of the change in the work function of the gate metal, as an additional influence quantity, the threshold voltage of the field-effect transistor and, therewith, the drain current, in the case of constant drain-source voltage, are changes, so that a quantitative determination of the gas is possible.
In a form of this sensor discussed in present German Published Patent Application 29 47 050, the gate electrode can be made of a very thin metal layer. This metal layer has islands which developed during its formation, which have not yet fully grown together. Apertures still remain between the islands, which enable the gas to pass through. In addition, the gate electrode can be made of a metal layer, which is provided with a pattern of bore holes of very small diameter, created, for example, by a laser ray, or by an electron or ion beam. Furthermore, the gate electrode can be designed as a grid electrode, or also be made of strips, arranged at a slight distance from each other.
The present invention provides an improved sensor for gases or vapors, which has an especially simple design and is provided with a sensitive sensor material.